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Direct Hydroxylation of Benzene to Phenol Over TS-1 Catalysts

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Direct Hydroxylation of Benzene to Phenol Over TS-1 Catalysts catalysts Article Direct Hydroxylation of Benzene to Phenol over TS-1 Catalysts Yuecheng Luo, Jiahui Xiong, Conglin Pang, Guiying Li * and Changwei Hu * Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China; [email protected] (Y.L.); [email protected] (J.X.); [email protected] (C.P.) * Correspondence: [email protected] (G.L.); [email protected] (C.H.); Tel./Fax: +86-28-8541-5745 (G.L.); +86-28-8541-1105 (C.H.) Received: 2 January 2018; Accepted: 22 January 2018; Published: 26 January 2018 Abstract: We synthesized a TS-1 catalyst to directly hydroxylate benzene to phenol with H2O2 as oxidant and water as solvent. The samples were characterized by FT-IR (Fourier Transform Infrared), DR UV-Vis (Diffused Reflectance Ultraviolet Visible), XRD (X-ray diffraction), SEM(scanning electron microscope), TEM (Transmission Electron Microscope), XPS (X-ray photoelectron spectroscopy), ICP (inductively coupled plasma spectrum), and N2 adsorption-desorption. A desirable phenol yield of 39% with 72% selectivity was obtained under optimized conditions: 0.15 g (0.34 to the mass of benzene) TS-1, 5.6 mmol C6H6, reaction time 45 min, 0.80 mL H2O2 (30%), 40.0 mL H2O, and reaction temperature 70 ◦C. The reuse of the TS-1 catalyst illustrated that the catalyst had a slight loss of activity resulting from slight Ti leaching from the first run and then kept stable. Almost all of the Ti species added in the preparation were successfully incorporated into the TS-1 framework, which were responsible for the good catalytic activity. Extraframework Ti species were not selective for hydroxylation. Keywords: benzene; hydroxylation; phenol; TS-1 catalyst 1. Introduction Phenol is an important chemical widely used in medicine, pesticides, plastics, and so on. The cumene method is the main process for phenol production in current commercial manufacturing, and this method has three steps from benzene to the target product [1–3]. The yield of phenol is not high, with disadvantages of high energy consumption and equipment corrosion. There is also an equimolar amount of acetone formed as a co-product [4,5]. The number of studies on the synthesis of phenol directly from benzene has been increasing, emphasizing environmental friendliness and economic feasibility. The one-step direct hydroxylation of benzene to phenol with hydrogen peroxide, which requires the activation of C–H bonds in the aromatic ring and the subsequent insertion of oxygen, has drawn much attention [6–9]. Several efficient catalysts with high activity and stability have been designed and tested [10–13]. TS-1, a Ti-containing MFI (Mordenite Framework Inverted) structure zeolite, first synthesized in 1983 by Taramasso et al. [14], has been found to display excellent catalytic activity in the oxidation of the aromatic ring, such as aromatic hydroxylation and cyclohexanone ammoximation [15–18]. A TS-1-catalyzed reaction can proceed under mild conditions with aqueous H2O2 as the oxidant so as to satisfy the demand of environmental protection. TS-1 suffers from an intracrystalline diffusion limitation on account of the micropores blocked by the reaction intermediates and by-products, and this limitation is most pronounced for a low temperature liquid phase reaction [19,20]. Thereby, research on TS-1 post-synthesis to extend the pore size has attracted much attention, where the pore-modified TS-1 catalyst may possess the advantage of a micro/mesoporous structure. Hierarchical zeolites possess Catalysts 2018, 8, 49; doi:10.3390/catal8020049 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 49 2 of 14 both the typical characteristics of zeolites and the properties usually related to amorphous materials, such as a crystalline structure and the presence of larger pores [21]. The practical applications of hierarchical zeolites have been greatly extended, and have become one of the excellent potential strategies to overcome the mass transport limitation in the catalyst [22]. In our previous work, an activated carbon catalyst was found to be efficient for the direct hydroxylation of benzene [23,24]. However, the repeated use of the activated carbon catalyst requires cumbersome treatments, and the yield of phenol was not high enough. The Fe-loaded ZSM-5 catalyst, with N2O as the oxidant in a hydroxylation reaction, usually had an excellent selectivity to phenol [25]. On hierarchical zeolite material post-treated ZSM-5, N2O oxidated benzene to phenol with good stability and selectivity [26]. There are some reports of benzene hydroxylation by a Pd-loaded membrane reactor, with H2 and O2 gases as co-reactants, which had good selectivity [27–29]. N2O as an oxidant requires a high reaction temperature, and O2 as an oxidant usually requires inert gas as a carrier. Therefore, H2O2 as a cheap and energy-saving oxidant is widely used in current research. Liya Hu et al. reported that vanadium-containing mesoporous carbon composites displayed superb catalytic activity, but the reuse was not good enough [30]. In the hydroxylation of benzene by the TM PQ Corporation commercial catalyst TS-PQ , the conversion of H2O2 was 19.4% and the selectivity of the phenol was 92% with an excess use of benzene [31]. The Fe-CN/TS-1 catalysts, with metal chloride as a precursor and a TS-1 zeolite as a support, could oxidize benzene to phenol in a biphasic water–H2O2/acetonitrile medium with visible light irradiation [32,33]. Therefore, the preparation of a catalyst with high catalytic activity and high stability is of great interest. In the present work, a TS-1 catalyst was synthesized with a modified method and used for benzene hydroxylation to phenol, with H2O2 as the oxidant and water as the solvent. Commercial TS-1 and S-1 were also used for comparison. 2. Results and Discussion 2.1. Catalytic Activity Test 2.1.1. Optimization of Reaction Conditions The effect of reactant concentration. The concentration of hydrogen peroxide was investigated, and the results are shown in Figure1a. With an increasing concentration of H 2O2, the yield of phenol (PH) increased, reached the maximum at the concentration of 0.20 mol/L, and then decreased. The yield of catechol (CAT), hydroquinone (HQ), and benzoquinone (BQ) monotonously increased. The high concentration of H2O2 led to the over-oxidation of phenol to dihydroxybenzene (DHB, namely, CAT and HQ) and benzoquinone [34], so the selectivity of the phenol did not change much when the concentration of H2O2 was less than 0.20 mol/L. Then, the selectivity of the phenol decreased gradually for the production of more DHB and benzoquinone. The effect of reaction temperature. The activity of the catalysts was tested at 50 ◦C, 60 ◦C, 70 ◦C, 80 ◦C, and 90 ◦C, and the results are shown in Figure1b. With an increasing reaction temperature, the yield of phenol enhanced, reached a maximum at 70 ◦C, and then decreased, whereas the yield of DHB increased monotonously. It has been reported that a low temperature is beneficial to restrain the over-oxidation of phenol [35,36]. When the reaction temperature reached 80 ◦C, the generation of by-products was significantly faster than the generation of phenol, which led to an obvious decrease of selectivity to phenol. The effect of reaction time. The reaction time was studied in the range of 25–65 min and the results are shown in Figure1c. The conversion of benzene and the yields of DHB and benzoquinone increased monotonically with time. However, the yield of phenol increased within 45 min and then decreased, and the selectivity of phenol was stable within 45 min and then decreased. A longer reaction time resulted in an increased formation of over-oxidation products [37], which reduced the yield and selectivity to phenol. Catalysts 2018, 8, 49 3 of 14 CatalystsThe 2018 effect, 8, x of FOR catalyst PEER REVIEW amount. The mass ratio of TS-1 to benzene was increased from 0.30 to30.39, of 14 and the results are shown in Figure1d. The yield of phenol increased with the mass ratio of TS-1 to benzeneup to 0.38, up and to 0.38,the yield and theof over yield-oxidation of over-oxidation products, products, DHB and DHB benzoquinone, and benzoquinone, had the same had thevariation same variationtrend. When trend. the mass When ratio the of mass TS-1 ratio to benzene of TS-1 was to benzene greater than was 0.39, greater the thanexcess 0.39, catalyst the excessmight catalyze catalyst the noneffective decomposition of H2O2, which was responsible for the decrease of benzene conversion. might catalyze the noneffective decomposition of H2O2, which was responsible for the decrease of benzeneConsidering conversion. the yield Considering and selectivity the of yield phenol, and selectivitya mass ratio of of phenol, TS-1 to a massbenzene ratio of ofaround TS-1 to 0.34, benzene namely of around0.15 g TS 0.34,-1, was namely rational. 0.15 g TS-1, was rational. Figure 1. The yield of products, the conversion of benzene, and the selectivity of phenol. Reaction Figure 1. The yield of products, the conversion of benzene, and the selectivity of phenol. Reaction conditions: 5.6 mmol C6H6, 40.0 mL H2O: (a) The effect of concentration of H2O2, mTS-1/mbenzene = 0.34, conditions: 5.6 mmol C6H6, 40.0 mL H2O: (a)◦ The effect of concentration of H2O2, mTS-1/mbenzene = 0.34, reaction time 45 min, reaction temperature 70 C; (b) The effect of reaction temperature, mTS-1/mbenzene reaction time 45 min, reaction temperature 70 °C ; (b) The effect of reaction temperature, mTS-1/mbenzene = 0.34, reaction time 45 min, 0.80 mL H2O2 (30%); (c) The effect of reaction time, mTS-1/mbenzene = 0.34, = 0.34, reaction time 45 min, 0.80 mL H2O2 (30%);◦ (c) The effect of reaction time, mTS-1/mbenzene = 0.34, 0.80 mL H2O2 (30%), reaction temperature 70 C; (d) The effect of catalyst amount, 45 min, 0.80 mL 0.80 mL H2O2 (30%), reaction temperature◦ 70 °C ; (d) The effect of catalyst amount, 45 min, 0.80 mL H2O2 (30%), reaction temperature 70 C.
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